Glycolysis: The Payoff Phase

Introduction

The investment phase feeds two molecules of glyceraldehyde 3-phosphate (GAP) into the same payoff sequence. Each GAP is oxidised and transformed into pyruvate while making one NADH and two ATP. The multiplication by two is the key to glycolysis's net balance. Following which phosphate comes from inorganic phosphate and which is donated to ADP also shows why ATP can be made directly from metabolic intermediates without a membrane or oxygen.

Core explanation

Glyceraldehyde-3-phosphate dehydrogenase oxidises GAP while reducing NAD⁺ to NADH. Inorganic phosphate, Pi, is incorporated to form 1,3-bisphosphoglycerate (1,3-BPG). This is not an ATP-consuming phosphorylation: the phosphate comes from the free Pi pool, while oxidation supplies the favourable chemistry needed to create a high phosphoryl-transfer-potential acyl phosphate. For each glucose, two GAP molecules undergo this step, producing two NADH.

Phosphoglycerate kinase transfers the high-potential phosphate at carbon 1 of 1,3-BPG to ADP, forming ATP and 3-phosphoglycerate. This is substrate-level phosphorylation because a phosphorylated organic intermediate directly donates the group to ADP. The enzyme runs once per GAP, so it generates two ATP per glucose, just enough to repay the two ATP invested earlier. Its name may reflect the reverse reaction under a naming convention; the direction in glycolysis follows actual concentrations.

Phosphoglycerate mutase moves the remaining phosphate from carbon 3 to carbon 2, producing 2-phosphoglycerate. Enolase then removes water to form phosphoenolpyruvate (PEP). The rearrangement creates a phosphoryl donor whose dephosphorylated product can undergo favourable tautomerisation to pyruvate. Neither mutase nor enolase directly makes ATP in these steps, but they prepare the second ATP-forming reaction.

Pyruvate kinase transfers PEP's phosphoryl group to ADP, giving ATP and pyruvate. Because two PEP arise per glucose, this generates two additional ATP. Total payoff production is four ATP and two NADH per glucose. Subtracting the two ATP invested gives a net of two ATP, alongside two pyruvate and two NADH. Oxygen is not a direct reactant in glycolysis, but continued flux requires recycling the NADH to NAD⁺ through respiration or fermentation.

The net tally assumes one glucose is the starting substrate and that the standard ten-reaction pathway runs to pyruvate. Entering glycolysis from glycogen or another sugar can change ATP accounting at the entry point. ATP equivalents from later NADH oxidation are not part of the direct glycolytic ATP count and depend on cellular shuttles and respiratory coupling.

Step-by-step reasoning

Begin with two GAP per glucose. For one GAP, mark one NAD⁺→NADH at the dehydrogenase step, one ATP at phosphoglycerate kinase and one ATP at pyruvate kinase. Multiply each by two. Subtract the two ATP consumed in the investment phase and verify that two three-carbon pyruvate molecules retain all six original carbons. Then state how NAD⁺ is regenerated for sustained flux.

Visual explanation

Draw two parallel GAP tracks leading to two pyruvate molecules. On each track place NAD⁺→NADH and Pi entry at 1,3-BPG, an ATP arrow at 3-phosphoglycerate formation, water release at PEP formation, and another ATP arrow at pyruvate formation. Combine the tracks in a balance box: 4 ATP produced − 2 ATP spent = 2 net ATP; 2 NADH produced.

Real-world analogy

An upfront two-token investment prepares two identical production lines. Each line then returns two tokens, for four returned and a net gain of two. The analogy helps counting, but the chemical value comes from oxidation, phosphate-transfer potential and the actual enzyme-coupled reactions, not from tokens appearing automatically.

Real-world example

Mature red blood cells rely heavily on glycolysis for ATP because they lack mitochondria. They can still make the direct two net ATP per glucose through substrate-level phosphorylation. Their NADH must be reoxidised for glycolysis to continue, commonly through lactate production, which the next page examines.

Why?

Why can the payoff phase make ATP without oxygen? ATP is formed by direct transfer of a phosphoryl group from 1,3-BPG or PEP to ADP. Oxygen is not a reagent in those transfers; oxygen availability affects the later route for reoxidising NADH, not the immediate substrate-level phosphorylation steps.

Common misconception

“Glycolysis makes four net ATP because the payoff phase makes four.” Two ATP were spent earlier. The correct net from glucose to pyruvate is four produced minus two consumed, or two, before considering any later use of NADH.

Worked example

If seven glucose molecules complete glycolysis to pyruvate, the investment steps consume 14 ATP. The payoff phase processes 14 GAP, producing 28 ATP and 14 NADH. Net ATP is 28 − 14 = 14, and 14 pyruvate molecules form. The calculation assumes no alternate entry or diversion and does not assign an ATP yield to NADH because its later fate has not been specified.

Quick check

1. Which two glycolytic enzymes produce ATP directly from phosphorylated intermediates? Answer: Phosphoglycerate kinase transfers phosphate from 1,3-BPG, and pyruvate kinase transfers phosphate from PEP; both are substrate-level phosphorylation.

Exam focus

Differentiate gross and net ATP and keep the two-GAP multiplier visible. Identify Pi entry at GAP dehydrogenase rather than ATP consumption. State that NADH generation requires NAD⁺ recycling and exclude later oxidative-phosphorylation ATP from direct glycolytic yield.

Advanced insight

The GAP dehydrogenase and phosphoglycerate kinase steps couple substrate oxidation to ATP formation through 1,3-BPG. They illustrate how energy can be conserved in a reactive acyl-phosphate intermediate instead of being released as heat. The actual direction of the kinase step remains governed by metabolite activities, despite its textbook arrow.

Summary

Each GAP is oxidised to yield one NADH and converted to pyruvate while producing two ATP by substrate-level phosphorylation. Two GAP per glucose give four gross ATP and two NADH. After subtracting the investment phase, glycolysis gives two net ATP per glucose, with NAD⁺ regeneration needed for continued operation.

Practice questions

1. How many ATP are made directly in the payoff phase from one glucose, and what is the net glycolytic ATP yield? Answer: Four ATP are made directly, two at phosphoglycerate kinase and two at pyruvate kinase. Two were spent earlier, leaving two net ATP. 2. Why does glyceraldehyde-3-phosphate dehydrogenase require Pi even though it produces no ATP directly? Answer: Pi is incorporated while GAP is oxidised to form 1,3-BPG, whose high phosphoryl-transfer potential enables the following kinase step to make ATP. 3. If NAD⁺ is unavailable, which payoff reaction is directly blocked and what follows for ATP production? Answer: GAP dehydrogenase cannot oxidise GAP, so the pathway cannot supply 1,3-BPG and downstream intermediates. Sustained ATP production from the payoff phase then stops unless NAD⁺ is regenerated.